The invention relates to measurement of size of objects in a high-throughput environment. An example is measurement of chip-scale package objects in an electronic circuit production environment.
As electronic circuits and electronic integrated circuit packages become increasingly miniaturised the need for accurate measurement and control of parameters in the manufacturing environment becomes increasingly important in the reduction of defects and, ultimately, in the reduction of costs associated with that process. This is particularly true of ball placement in ball grid array, chip scale package and flip chip manufacturing.
In these processes the solder balls of a specified diameter are placed at a location on the copper pad of a packaged integrated circuit. Following the placement process the solder balls are attached to the substrate in a reflow process. The accuracy requirements in the position and height of the solder balls following this placement and reflow process depend on the specific dimensions of the package. In general. however, solder ball height and the relative heights of all the solder balls on a package is critical to the performance of the final package when it is in a functional position on a circuit board. For example, if one solder ball in a package is sufficiently far below the average height of all other solder balls it is likely that this ball will form an open circuit with the board at this location.
It is known to provide a system to measure x, y, and z positional and size data of a BGA using a two-camera xe2x80x9cstereoxe2x80x9d arrangement, as described in European Patent Specification No. EP0638801B1 (IBM). One camera is used to measure the centrality of a BGA and the other to sense the flatness. The flatness is measured by a tilted camera sensing crescent-shaped reflections from a light source located opposite the camera.
It appears that the accuracy which can be achieved for the height dimension is limited by variations in the sizes of the images reflected from the balls. Also, it appears that the throughput would not be as high as desirable because of the need to capture multiple images for each BGA. Also, capturing of multiple images (successive juxtaposed rows) for a single object requires sophisticated work table robotic control and image processing synchronisation.
One object of the invention is to provide for improved throughput in such systems.
Another object is to provide for improved accuracy.
According to the invention, there is provided a measurement system comprising means for supporting an object to be inspected in an object plane, a light source mounted to illuminate the object, an obliquely-mounted camera for capture of images of the illuminated object, and an image processor comprising means for determining height of the object according to measured offset from a calibration image, characterised in that,
the camera comprises a sensor and a separate lens, and
the system further comprises means for supporting the sensor and the lens at mutually different angles whereby a plane through the sensor and a plane through the lens substantially intersect at the object plane according to the Scheimpflug principle to provide a relatively large field of view in focus.
In one embodiment, the system further comprises a reference camera mounted to capture a two-dimensional normal image of the object, and the image processor comprises means for determining the calibration image from said normal image.
In one embodiment, the light source is mounted with respect to the obliquely-mounted camera whereby the peak of the object is sensed.
In one embodiment, only an area including the peak of the object is sensed.
In one embodiment, the image processor comprises means for determining a centroid of the image to determine the peak of the object.
In one embodiment, the system further comprises a lateral light source arranged in an annular configuration with respect to the object support means to provide an annular image for the reference camera, and the image processor comprises means for determining the centroid of the annular image to provide the normal image.
In one embodiment, the obliquely-mounted camera comprises a filter to filter out light from the lateral light source, and the image processor comprises means for controlling simultaneous image acquisition from both the normal camera and the obliquely-mounted camera.
In one embodiment, the lateral light source comprises a ring of LEDs.
In another embodiment, the image processor stores reference data for a ball grid array and means for accepting or rejecting a ball grid array according to comparison of the measured height data and the reference data.
According to another aspect, the invention provides a method of inspecting an object comprising the steps of:
mounting a primary camera sensor and a lens over an object space so that they have non-parallel planes,
mounting a reference camera over the object space,
causing relative movement of the cameras and an object to a stationary position at which the primary camera lens plane, the primary camera sensor plane, and the object plane intersect according to the Scheimpflug principle,
capturing a primary image of the full object at the primary camera and capturing a reference image of the full object at the reference camera without further relative movement of the cameras and the object, and
an image processor determining a calibration image from the reference image, and determining height of the object according to a measured offset of the primary image from the calibration image.
In one embodiment, the object is a ball grid array.
In one embodiment, the object is illuminated with lateral light directed substantially parallel to the object plane whereby the reference image is annular, and the object is also illuminated by oblique light directed from an oblique angle whereby the primary image corresponds to a surface area only including an object peak.
In one embodiment, the lateral light and the oblique light have different wavelengths, light entering a camera is filtered to exclude the light for the other camera, and both the primary and the reference images are simultaneously captured.